2.25 gpm at a 60F rise requires an output at the boiler of 70,000 btuh. Two such used at the same time requires 140,000 btuh output. If you need a smaller rise for showers from 45F input to 105 output, that is a 60F rise, you could use a 105, but you'll need the larger size at 140,000 output if you must.

Baxis are widely used in the UK.http://www.ciao.co.uk/Baxi_Combi_Boiler__5318367

As with the post below, there are no agencies that do longitudinal studies of boilers, so you get political answers from salesmen and satisfied or unsatisfied anecdotes from owners.

Customers are often dissatisfied as hard water will plug thm in a couple of years, so they need yearly overhauls.